Japanese Patent Application Laid-Open No. 2008-078086 discloses a vehicle light having a light emitting diode (LED) as a light source and a light guide for controlling the light distribution pattern of light beams from the LED. FIG. 1 is a vertical cross sectional view illustrating the configuration of a conventional vehicle light. As shown, the vehicle light has a light source 100 including a light emitting device 100a facing upward. A light guide 102 is disposed above the light source 100. The light guide 102 includes a light incident surface 104, a reflecting surface 106, and a light exiting surface 108. Light beams emitted from the light source 100 can enter the light guide 102 through the light incident surface 104. The reflecting surface 106 is disposed near the rear side of a vehicle body and the entering light beams can be reflected by the reflecting surface 106 to be directed in the forward direction of the vehicle body. The reflected light beams exit through the light exiting surface 108 disposed near the front side of the vehicle body.
The light guide disclosed in Japanese Patent Application Laid-Open No. 2008-078086 is made of a glass material, and in order to decrease the entire weight of the vehicle light, the inventors examined the light guide that was prepared by using a transparent acrylic resin. In this case, it was observed that color blurring occurred at the boundary of the light distribution pattern. When the acrylic resin was replaced with polycarbonate having a higher heat resistance than acrylic resin, it was observed that the color blurring (color shading) significantly occurred at the bright-dark boundary of the light distribution pattern.
When the light source utilizes an LED, the inside temperature of the vehicle light is increased by the heat generated from the LED, and accordingly, it may be helpful to form the light guide and the like from a high heat resistant, transparent material such as polycarbonate. However, the polycarbonate material can have variable refractive indices depending on the wavelength of entering light beam when compared with other transparent resin materials, resulting in occurrence of large chromatic dispersion. It should be noted that the chromatic dispersion means the dispersion of light of which phenomenon can occur for a material having various refractive indices depending on wavelengths of incident light beams.
Accordingly, if the light guide for forming a predetermined light distribution pattern is formed from such a polycarbonate material with large chromatic dispersion, the color blurring can be generated by projecting light beams with particular wavelengths due to the chromatic dispersion, at areas outside of the bright-dark boundary of the light distribution pattern. Accordingly, there is the problem in which the illumination light may have color shading.
If the illumination light has such color shading, when an object with monochromatic color is illuminated therewith, the object can be observed with different colors at different positions, thereby degrading the color rendering properties. Such color shading of illumination light due to chromatic dispersion may occur not only in the case where the polycarbonate material is used, but also in the cases where other transparent materials including glass, acrylic resin and the like are used for molding a light guide although the degree of occurrence may vary.